1. Field of the Invention
This invention relates generally to a system and method for discharging a high voltage vehicle battery following a crash or other vehicle damaging event and, more particularly, to a system and method for discharging a lithium-ion vehicle battery following a crash or other vehicle damaging event that includes dissipating cell voltages in the battery using cell balancing resistors.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV), that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead-acid, etc. A typical high voltage battery system for an electric vehicle may include several battery cells or modules to provide the vehicle power and energy requirements, where each battery module may include a certain number of battery cells, such as twelve cells. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
During a vehicle crash or other vehicle damaging event, the rearrangement and/or damage to various vehicle parts and the like may cause unwanted electrical connections and the discharge of various fluids from the vehicle to occur. Because of the electrical power that may be stored in the vehicle battery, these things could cause potentially hazardous situations, such as electrifying of vehicle parts, chemical hazards, fire hazards, etc.
Fault isolation detection systems are sometimes employed in electrical circuits to provide fault detection. Electric vehicles are one electrical system that typically employ fault isolation detection systems to prevent a person from being harmed by the system.
In order to provide electrical fault isolation, some vehicles are equipped with a battery disconnect unit (BDU) that automatically disconnects or removes battery power from the vehicle after a crash event by opening battery contactors. Also, it is known in the art to employ a manual service disconnect (MSD), which is a device that separates a battery into two parts, where a trained responder responding to a vehicle crash could remove the MSD to isolate the battery. However, these known isolation techniques do not remove the charge from the battery, only contain it.
As mentioned above, a high voltage battery for a vehicle typically includes several battery cells electrically coupled in series. As a result of many factors, such as internal cell resistance, electrical connections, battery aging, etc., the state-of-charge (SOC) of the cells in the battery may drift apart during operation of the battery over time. A battery management system may be provided to monitor the state-of-charge of each battery cell and the temperature of the battery, and control how much the battery can be charged and discharged based on the state-of-charge of the maximum charged cell and the minimum charged cell. The battery cannot be used to provide power if the cell with the lowest state-of-charge drops below some minimum state-of-charge because that cell may become damaged, and the battery cannot be over-charged beyond some maximum state-of-charge for the cell with the highest state-of-charge because that cell may become overheated and damaged. Thus, a cell with a low state-of-charge may prevent the battery from being used even though the other cells may have a suitable or significant state-of-charge.
It is known in the art to provide a cell balancing system that attempts to control the charging and discharging of the cells within a high voltage vehicle battery so that they have about the same level charge at any given point in time. In one known cell balancing circuit, cell balancing resistors are provided in parallel with the battery cells that allow the cells to be electrically dissipated to a desired level so that the charge matches other battery cells.